In recent years, secondary batteries have been increasingly used. In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is very high.
In addition, secondary batteries may be classified based on the structure of an electrode assembly having a positive electrode/separator/negative electrode structure. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which a long sheet type positive electrode and a long sheet type negative electrode are wound while a separator is disposed between the positive electrode and the negative electrode, a stacked type structure in which pluralities of positive electrodes and negative electrodes having a predetermined size are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes, or a stacked/folded type structure in which predetermined numbers of positive electrodes and negative electrodes are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a bi-cell or a full-cell and then a plurality of bi-cells or full-cells is folded using a separation film.
In recent years, much interest has been taken in a pouch-shaped battery configured to have a structure in which such a stacked or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of an aluminum laminate sheet because of low manufacturing costs, light weight, easy modification in shape, etc. In addition, the use of such a pouch-shaped battery has gradually increased.
FIG. 1 is an exploded perspective view typically showing a general structure of a conventional representative pouch-shaped secondary battery.
Referring to FIG. 1, a pouch-shaped secondary battery 10 includes an electrode assembly 30, electrode tabs 40 and 50 extending from the electrode assembly 30, electrode leads 60 and 70 connected respectively to the electrode tabs 40 and 50 by welding, and a battery case 20 for receiving the electrode assembly 30.
The electrode assembly 30 is a power generating element including positive electrodes and negative electrodes sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes. The electrode assembly 30 is configured to have a stacked type structure or a stacked/folded type structure. The electrode tabs 40 and 50 extend from corresponding electrode plates of the electrode assembly 30. The electrode leads 60 and 70 are electrically connected to the electrode tabs 40 and 50, extending from the corresponding electrode plates of the electrode assembly 30, respectively, for example, by welding. The electrode leads 60 and 70 are partially exposed outward from the battery case 20. In addition, insulating films 80 for improving sealability between the battery case 20 and the electrode leads 60 and 70 and, at the same time, securing electrical insulation between the battery case 20 and the electrode leads 60 and 70 are partially attached to the upper and lower surfaces of the electrode leads 60 and 70.
The battery case 20 is made of an aluminum laminate sheet. The battery case 20 has a space defined therein to receive the electrode assembly 30. The battery case 20 is formed generally in the shape of a pouch. In a case in which the electrode assembly 30 is a stacked type electrode assembly as shown in FIG. 1, the inner upper end of the battery case 20 is spaced apart from the electrode assembly 30 such that the positive electrode tabs 40 and the negative electrode tabs 50 can be coupled to the electrode leads 60 and 70, respectively.
For the pouch-shaped battery with the above-stated construction, however, at the step of receiving the electrode assembly in the battery case made of the laminate sheet, injecting an electrolyte into the battery case, and sealing the battery case by thermal welding, a thermally welded portion (a sealed portion) may be contaminated by the electrolyte, and it is difficult to maintain a completely sealed state even after thermal welding due to excessive welding of the innermost resin layer of the laminate sheet and/or outward protrusion of the inner resin layer caused by pressurization. As a result, moisture may easily permeate into the battery case, and the electrolyte may leak from the battery case.
In addition, for the pouch-shaped battery, an insulation breakdown phenomenon may occur due to exposure of the metal layer at the end of the laminate sheet of the battery case, and moisture may penetrate into the battery case through the thermally welded portion of the end of the laminate sheet of the battery case.
In connection with this case, conventional batteries are disclosed in which a PET label or tape is attached to the outer edge of the thermally welded portion to achieve insulation. In a case in which the outer edge of the thermally welded portion is insulated using the PET label or tape, however, the label or the tape may be separated from the outer edge of the thermally welded portion, or air bubbles or wrinkles may formed in the PET label or tape.
Korean Patent Application Publication No. 2001-0078364 discloses a pouch-shaped battery in which an ultraviolet (UV) curable agent, as an auxiliary sealing agent, is applied to an outer edge of a thermally welded portion and is then cured, thereby improving sealability of the battery. However, it is not easy to apply the UV curable agent, which has predetermined viscosity and mobility, to the outer edge of the thermally welded portion, which is thin in vertical section, and the UV curable agent may run down when UV light is applied to cure the UV curable agent after application of the UV curable agent with the result that an effect of improving sealability of the battery is reduced. In conclusion, the above-described technology is not suitable to be applied to a real mass production process.
In addition, Korean Patent Application Publication No. 2001-0004352 discloses a method of manufacturing a sealed battery including applying a UV curable resin to a gasket sealed portion of a positive electrode cap, a safety valve, a welded portion between the positive electrode cap and a battery case, and a sealed portion of an electrolyte injection port and curing the UV curable resin. However, the above-described technology is applied to a cylindrical battery or a prismatic battery. It is difficult to apply the above-described technology to the pouch-shaped battery having problems in that it is troublesome to apply the curable material, and the curable material may run down at the sealed portions as previously described due to structural characteristics of the pouch-shaped battery.
Therefore, there is a high necessity for a technology that is capable of fundamentally solving problems caused in a case in which a curable agent is used to improve sealability of a thermally welded portion of a pouch-shaped battery and to prevent the occurrence of an insulation breakdown phenomenon.